Rewrite boilerplate code in buffer.rs

[Gnurou]

Per @bgrzesik on #10:

We could use a private trait in favor of BufferType enum. Something like:

trait BufferElement: Into<Self::RawType> {
    const TYPE: bindings::VABufferType::Type;
    type RawType;

    fn raw_element_size() -> usize {
        std::mem::size_of::<Self::RawType>()
    }
}

Replace tuple structs with strongly typed and documented copy of raw struct. It would have to be converted to raw when added to the picture (or prior vaRenderPicture), but during this step we could clone Rc<>s to dependencies (like surfaces and EncCodedBuffer) to Picture<> so those wouldn't get dropped before sync. It would clean up some cros-codecs code.

Also Picture::add_buffer could accept a trait that would wrap our use cases. Like so:

trait BufferContents {
    const TYPE: bindings::VABufferType::Type;

    fn element_size(&self) -> usize;

    fn num_elements(&self) -> usize;

    unsafe fn data_ptr(&mut self) -> *mut std::ffi::c_void;
}

// Or just use slice somehow
pub struct BufferArray<T>
where
    T: BufferElement,
{
    array: Vec<T::RawType>,
    // ... deps
}

impl<T> BufferContents for BufferArray<T>
where
    T: BufferElement,
{
 // ...
}

impl<T> BufferContents for Box<T> // Or just T
where
    T: BufferElement,
{
 // ...
}

impl BufferContents for SliceData
{
 // ...
}

In this way we could handle buffer arrays and single element buffers transparently.
On top of that we maybe could look into doing some validations.